The invention relates to a sorption pumping device, provided with a reservoir for sorption material which can be made to communicate, via an inlet duct, with a space to be evacuated. Sorption pumping devices of the subject kind are known as appears from British patent specification Nos. 921,666 and 939,395.
Known sorption pumping devices have a single, fixed inlet duct. A drawback of this construction is that, particularly at lower starting pressures in the space to be evacuated, effective use is only made of the layers of sorption material which are situated in the immediate vicinity of the opening of the inlet duct in the reservoir. These layers are most favourably situated with respect to the gases to be pumped from the space to be evacuated. Consequently, the these layers obtain a comparatively dense population of gas molecules during pumping, while the population of the remaining layers, situated further from the inlet opening, remains comparatively small. The latter is also due to the fact that the gas diffusion from layers with dense population to layers of sparse population is low at the low operating temperature of the sorption material (usually liquid nitrogen temperature).
Because only a part of the sorption material is effectively utilized, the pump as such is comparatively quickly saturated so that a frequent reactivation of the sorption material is required. Moreover, comparatively long pumping times are required in order to achieve a given minimum pressure, for example, the starting pressure for getter-ion pumps. Furthermore, it is difficult to realize low final pressures in the space to be evacuated.
So as to achieve a more effective use of the sorption material which is situated further from the inlet opening, it has been proposed (Review of Scientific Instruments 34, May 1963, pages 587-588) to sub-divide the sorption material into separate layers distributed over a large number of trays which are arranged in the reservoir. The gas which is sucked in is then more uniformly distributed between the sorption material surfaces on the various trays.
A pumping device of this kind, however, not only has a complex and expensive construction, but its action is not optimum either. The latter is mainly due to the fact that during one and the same pumping cycle from high to low pressure, the gas flow is always applied to the surface of the sorption material along the same course.